1. Field of the Invention
The present invention generally relates to a bidirectional power converter and, more particularly, to a cycloconverter which is capable of acting as both an inverter (converting DC power to AC power) and as a rectifier (converting AC power to DC power).
2. Description of the Prior Art
Bi-directional inverters/chargers are increasingly used in line-interactive uninterruptible power systems (UPSs), battery-backup stand-alone inverter systems, and alternative energy systems such as wind power and photovoltaic applications. A simplified block diagram of such a system is shown in FIG. 1. When voltage the bi-directional inverter/charger 2 functions as an inverter, it converts the dc voltage, V.sub.b, into an ac output voltage, V.sub.0, at line frequency to supply loads with various power factors. The dc source can be a low-voltage battery, or an alternative energy source with battery backup. Once the alternative ac source of V.sub.g, which can be standalone engine-generator set 4, or a utility line, is available, it will be used to supply the load power 6 with the activation of the transfer switch 8, S.sub.t, at the same time the bi-directional inverter/charger switches to charger operation to replenish the battery. An electromagnetic interference (EMI) filter 10 is also included. It is usually preferred for the converter to absorb sinusoidal current from the ac source when it operates as a charger to render better utilization of the available ac capacity.
The functions of such bi-directional inverter charger can be realized with a bi-directional dc/dc converter 12 in cascade with a four-quadrant full-bridge inverter/rectifier 14 as shown in FIG. 2A. A high-frequency (HF) transformer line T.sub.r is usually required to provide electrical isolation and voltage matching between the input dc and output ac voltages. In this kind of two-stage schemes, three HF inverters/rectifiers of either full-bridge, half-bridge or push-pull topology are needed and the power flow in either direction is always processed twice. In addition, extra dc-link filtering components are also a necessity.
In the past decade, single-stage, cycloconverter-based schemes as shown in FIG. 2B have constantly been sought. As shown, these comprise a high frequency inverter/rectifier 16 connected to a cycloconverter 18 through transformer T.sub.r. The cycloconverter-based bi-directional inverter/charger topology was disclosed in U.S. Pat. No. 4,742,441 to Akerson, herein incorporated by reference. Since then, different pulse width modulation (PWM) control methods have been developed to either suppress the transient voltage in the cycloconverter part, achieve reliable four-quadrant operation, or improve the dynamic performance of the converter.
The concern about reliable bi-directional operation has been looming large for the cycloconverted-based single-stage inverters/chargers. It stems from two basic topological traits of the converter, i.e. the lack of self-present current freewheeling paths inside the cycloconverter because all of the switches are bi-directional and need control to activate in both directions, and the intrinsic transient voltage appearing on the cycloconverter switches during boost mode operation when power is transferred from the output (ac side) to the input (dc side). The former can be solved by the application of proper PWM sequence which ensure the existence of the output current freewheeling path while without shortening the transformer secondary winding. The latter is akin to any isolated boost-type of converters, and has to be solved with extra voltage clamping circuitry. One such example is shown in FIG. 3 using clamping circuitry. However, as discussed herein below, this arrangement has severe limitations which affect its performance and desirability.